Portable battery-powered wireless capability is becoming an expected feature for many types of electronic sensing devices in general and for portable digital radiographic imaging detectors in particular. With medical imaging receiver equipment, portable, untethered operation offers some promise of improved patient care, with advantages including improved operator workflow and equipment adaptability.
Digital radiographic (DR) detectors, also known as flat panel detectors (FPDs) have revolutionized the field of general radiography by providing the capability to rapidly visualize and communicate X-ray images. Patient X-rays can be efficiently transmitted via data networks to one or more remote locations for analysis and diagnosis by radiologists without the delay incurred when sending physical films through the mail or via couriers to reach remotely located radiologists.
FIG. 1 shows a partial cutaway view of the basic imaging components of a conventional FPD. The FPD generally includes a large-area two-dimensional image sensor array 10 having many thousands of radiation-sensitive pixel sites 14 that are arranged in a matrix of rows and columns. Each pixel site 14 has one or more photosensors 12, such as a PIN photodiode, and one or more switch elements 16, such as thin film transistors (TFTs). As is generally understood, the photosensors convert X-ray radiation into signals that are read out by the switch elements 16 and stored in a memory associated with the detector. This conventional DR arrangement allows each radiation-sensitive pixel site 14 to be individually addressed and read out using conductive metalized rows and columns that extend across the length and width of the detector panel.
Radiation-sensitive pixel sites 14 of the FPD typically use photodiodes such as PIN photodiodes, but other photosensor technology can also be used. When photodiodes are used for radiographic image sensing, the X-ray radiation is first converted to a wavelength suitable to the photodiode at each radiation site. This is conventionally done using a scintillator screen 15 that, upon stimulation by X-ray radiation of one wavelength, emits photons in a second wavelength that is within the sensitivity of the photodiodes. Each photodiode then produces an electric charge that is proportionate to the number of photons it receives. The process of detecting X-ray radiation in this way, converting the detected radiation to digital information, and storing the digital information internally is herein termed image acquisition. Once the X-ray image has been acquired, it can be transmitted from the FPD to an operator console for image evaluation, downstream distribution, and/or long term storage.
In conventional, large-scale digital radiographic installations, the FPD is permanently installed at a predetermined location used for patient imaging. This type of installation is typically set up for obtaining a standardized set of radiological images that are routinely needed for a large number of patients. However, for situations where non-standard images are required, the patient is positioned relative to the stationary DR detector. For some patients, this creates a problem that is not easily resolved with digital radiography and can even necessitate return to the use of older technologies, such as the use of a phosphor computed radiography (CR) X-ray cassette. This can result in added cost and inefficiency and forces a medical facility to maintain older equipment to handle types of imaging not readily performed on the DR system.
The portable, cassette-type FPD provides an alternate solution to this positioning problem and allows for smaller and more portable x-ray imaging systems. A portable FPD enhances the efficiency of operator workflow since the detector can be readily positioned behind the patient, rather than requiring the patient to take an awkward position for imaging. In many cases, an FPD can replace the need for multiple detectors, since the same detector can be used both in a wall-mount position and a horizontal table position. The portable FPD has the flexibility to be easily and quickly moved to any suitable location throughout a DR suite and yet still provides immediate access to the acquired x-ray image.
Portable cassette-type detectors have been enabled by state of the art advances in both electrical components and packaging, allowing significant reduction in overall size and weight. A cassette-type FPD has been described, for example, in U.S. Pat. No. 5,844,961, which generally describes a filmless digital X-ray cassette having external dimensions approximately equal to those of a standard sized X-ray film or CR cassette. A combined communications and power link cable or tether serves both as a means to transfer digital image data from the FPD as well as to supply power to the flat panel device. An external AC to DC power supply also connects to the cassette through this combined communication and power link. The power supply, such as a battery, may alternatively be located inside the cassette to overcome the liability of needing a direct cable link for this purpose.
U.S. Pat. No. 7,015,478 entitled “X-ray Imaging Apparatus” (Yamamoto) describes a portable electronic cassette-type detector with an interconnecting cable that provides both communication and power. That patent describes attaching a second cable to the cassette, to connect and disconnect the device when positioning the detector under the patient. A battery and power supply can be located inside the detector housing.
Tethered solutions such as those presented in the McEvoy et al. and Yamamoto patents have inherent disadvantages. Connection of the interconnecting cables is made and maintained at each end, which can be difficult to achieve when moving the FPD around and behind some portion of the patient. The tether becomes a significant encumbrance when trying to optimally position a cassette-type FPD under a patient. The tether is also a potential source for damage to the sensing device because of the likelihood of inadvertently catching or tripping over the cable while moving the FPD to a new location. The tether also limits how far the detector can be from the console. Yet another problem relates to the need for multiple DC voltage levels for different portions of the sensing and processing circuitry. For these reasons, there can be particular difficulties in tethering DR imaging panels.
To effectively eliminate tethered power supplies, there is the need for portable on-board power that is compact, lightweight, and allows a runtime of several hours. High-energy lithium polymer batteries, typically with two or more cells connected in series, for example, may supply power sufficient for the complex communication, control and imaging circuitry on a portable FPD. A switch-mode power supply, (SMPS,) is a DC to DC converter that can use a battery source and is capable of producing output voltages that can be less than or greater than the voltage supplied by the battery. There are a number of types of DC to DC converter topologies typically used for SMPS devices and familiar to those skilled in the electronics art. Examples of a few of these topologies are buck, boost, SEPIC, CUK, flyback, and forward converters.
The SMPS operates by periodic switching of current into inductors and capacitors that serve as energy storage elements. Because their energy storage and switching components can be relatively small, SMPS devices are comparatively compact and lightweight. At the same time, SMPS devices are capable of power conversion efficiencies of up to 95 percent.
Although SMPSs offer these advantages, there can also be significant drawbacks. Among these drawbacks are inherently high noise levels when compared with linear power regulators and other power supply types. The noise generated from SMPS switching can be both conducted and radiated and can cause significant interference and image artifacts, degrading the performance of other nearby apparatuses, subsystems, or circuits, especially with regard to signal to noise ratio (SNR). This effect can be particularly pronounced for sensitive equipment such as that of a DR detector, with its high-impedance detector circuitry packaged in close proximity to inductors on the SMPS.
The main types of electromagnetic inductance (EMI) from switching power supplies are radiated electric and magnetic fields, generated in close proximity to switched components. A number of conventional solutions have been used for minimizing EMI effects with SMPSs. For conducted EMI modes, additional filter elements can be used, added in series along conduction paths that lie near power supply input and output lines. These filter elements typically include capacitors and series ferrite inductors that shunt or absorb the high frequency energy before it conducts to adjacent circuits.
Another method for reducing conducted EMI, often in conjunction with the use of noise filter elements, is to synchronize the switching frequency of one or more of the switch-mode power supplies to the master internal clock or other timing waveform that is already employed in an electronic device. For example, timing waveforms may trigger sensitive operations needed for sample-and-hold measurement, charge transfer, and small-signal analog-to-digital (A/D) conversion. When all switch-mode supplies for an apparatus are synchronized with or run on a common clock, filter implementation is simplified because the interference noise is constrained to one common frequency band. Using switch-mode supplies synchronized to a master system clock, the timing of transient noise from the switch-mode PWM waveform can be adjusted to prevent these transients from occurring during the sensitive device operations. As one example of a technique for timing synchronization, U.S. Pat. No. 4,034,232 describes a method of synchronizing multiple power supplies and positionally phase-shifting the individual clocks to minimize disruptive transients.
Mitigation techniques for radiated EMI propagation are more difficult and expensive, since the radiated noise can be from many different sources proximate to the EMI sensitive circuit or subsystem. Conventional solutions for reducing radiated EMI include protecting sensitive circuit components by shielding. Since radiated EMI has both an electric and magnetic component, two types of shielding are employed. Ground planes and Faraday enclosures have been used for E-field shielding, effectively shunting the electric field and significantly reducing it. For the magnetic H field component, thick ferromagnetic materials with high permeability, such as Mu metal, a nickel-iron alloy, have been used to shunt stray magnetic flux and to keep it from coupling into sensitive conductor traces in nearby circuits.
Although SMPSs can be packaged to fit within the narrow confines of a portable DR detector, integrating these noisy power supplies into the detector housing without introducing interference can be particularly challenging. The need for protection from conducted and radiated EMI that is SMPS-generated can add significantly to the size and weight requirements of an untethered DR detector. Added filter elements for conducted noise compensation increase the overall cost, size, and complexity of the SMPS. Conventional H-field shielding solutions for radiated noise, including Mu metal, are ineffective at the high switching frequencies used. Even if a suitable shielding material could be found, shielding can significantly increase device size and weight.
Thus, there is a need for an improved digital imaging detector that includes an on-board SMPS power supply, but that doesn't suffer from image degradation resulting from EMI.